Ferrous metal die casting process and products



cf. 6, 1970 -r0 ETAL 3532 63 FERROUS METAL DIE CASTING PROCESS AND PRODUCTS,

Filed May 11, 1967 6 Sheets-Sheet 1 ITWVTTlTO Ronatd L. Bar to DaLLas T. Huvd FERROUS METAL DIE CASTING PROCESS AND PRODUCTS Filed may 11, 1967 L A m 0 T R A B L R 6 Sheets-Sheet 5 [2 5a. 0/5 0057 Lg 5b. sn/vp c057 GPHY [EON-05 6457-200X X 0 0 5 h u m W N 0 M w H W 710. SAND M57 M41. LEHBLE neo/vflA/NEHLED-ZOO x 1% 7a. 0/5 mar O Uu e Tau. T O H O j k @i A wfiww a. I In m T U b ct 6, 19?@- R-r0 ET AL 3,532, FERROUS METAL DIE CASTING PROCESS AND PRODUCTS Filed may 11, 1967 6 Sheets-Sheet 4.

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Invervtovs: Ronald. L. BaT- 'to Dattas T. Hw'd b9 W2 Their A t tow-neg United States Patent 3,532,561 FERROUS METAL DIE CASTING PROCESS AND PRODUCTS Ronald L. Barto, Wicklitfe, and Dallas T. Hurd, Gates Mills, Ohio, assignors to General Electric Company, a

corporation of New York Filed May 11, 1967, Ser. No. 637,814 Int. Cl. C21d 9/00; B22d 27/04 US. Cl. 14837 10 Claims ABSTRACT OF THE DISCLOSURE A process for producing ferrous metal objects by pressure injection die casting at high temperatures in molds comprising or lined with or having inserts of refractory metals, particularly tungsten, molybdenum or their alloys. Due to the high rate of heat extraction, even with the molds operating at temperatures elevated enough to prevent premature freezing and poor cast surfaces, the refractory metal surface mold produces a quenching effect resulting in extremely fine as-cast grain structures which are unusually susceptible to heat treatment to beneficially modify the structure. Specific improved types of products of the invention include stronger gray cast iron as well as white cast iron which can be heat treated in an unusually short period of time to malleablize the structure, graphitizing essentially all of the carbide content and yielding a stronger product. Low carbon steel and low alloy steels produced by this process have unusually fine grain size and an unusual susceptibility to heat treatment for further refining of the cast grain size. This results in improved properties including strength. The fine grain size of stainless steels cast by this process increases corrosion resistance by providing more grain boundary area to dilute grain boundary impurities which are normally attacked in the corrosion of such steel.

CROSS-REFERENCES TO RELATED APPLICATION Application Ser. No. 718,640 was filed Apr. 3, 196 8, as a continuation-in-part of this application to apply to the casting of cast iron and the resulting products.

BACKGROUND OF THE INVENTION This invention relates to processes for casting ferrous metals and articles produced by such processes. More particularly, it relates to processes for producing precision parts or articles of commerce of ferrous metals by pressure injection die casting in relatively permanent molds.

The traditional method of casting ferrous metals into useful objects, which has been used for thousands of years and which still is the principal method for casting iron and steel today, is the pouring of molten metal by gravity transfer from a ladle or other container into a cavity formed within a bed of compressed sand or other refractory oxide material. In such casting processes, the molds are commonly destroyed during removal of castings. This leads to relatively high costs since the molds must be remade each time a casting is produced. Also, although the surfaces produced in the casting by the sand mold surface are often satisfactory for relatively large and imprecise objects, they are rather rough and they are not satisfactory for use in precision parts. Machining is often necessary to produce satisfactory flat and smooth surfaces in certain areas of large, coarse castings. At least some costly machining is often necessary in the production of even small sand cast ferrous metal parts.

In order to more accurately produce precision parts, processes such as investment casting and shell-mold casting have been developed. These processes utilize resinbonded and or cemented oxide ceramic formed in a thin shell about a dispable pattern such as of wax or frozen mercury which can be removed readily by melting. The shell can then be placed in a loose backing or support material such as sand before the casting is poured into the shell. As with conventional sand mold casting, the costs per casting are commensurately high.

In the efforts to produce castings more efficiently, a further step is permanent-mold casting utilizing molds open to the atmosphere. Although this process is sometimes inaccurately called die casting, it is distinguished from pressure injection die casting which uses closed molds and high pressure during injection and freezing of the molten cast metal. Permanent-mold casting as used with ferrous metals normally involves a mold or mold lining of carbonaceous material. However, due to the mechanical and thermal characteristics of such material, it is not suitable for very fast repetitive casting at high temperatures and exposure to rapidly-flowing liquid metal under substantial pressures.

These casting methods of prior art all have in common a more or less protracted mold filling time, during Which the molten metal being cast may begin to freeze sequentially in zones before the filling process is completed. Contributing to this phenomenon is the use of thermally insulating material with relatively poor heat conductivity as the mold material. Thus, the solidification process is considerbaly extended in time and may last for a number of minutes or even hours with large castings. It is known that, during such extended cooling processes, the kinetics of freezing will vary significantly from point to point Within the solidifying metal; zones with undesirably different metallurgical structures and different metallurgical properties may occur in different sections of the casting, such as by differential segregation of dissolved alloying elements. Also, the cast metal grain size Will be variable and relatively large. To accommodate such inflexibilities in the casting methods, conventional ferrous metal casting imposes narrow limits on the permissible chemical compositions of the metal being cast, and the attainment of optimum mechanical properties in the cast metal is diflicult at best and often impossible.

Conventional pressure die casting processes developed for casting objects of aluminum, zinc, copper, and various alloys based on these metals cannot satisfactorily be utilized for producing ferrous metal die castings for several reasons. Pressure die casting dies are normally made from tool steels such as the one known in the art as H-13 die steel. Temperatures necessary for casting the various cast irons and steels are so high that tool steel dies would not only have to pass through phase transition temperature ranges but would also be at such high operating temperatures as to be very weak and entirely unsatisfactory for extended casting production at high pressures, such as about 10,000-30,000 pounds per square inch, which might normally be used to obtain rapid and complete filling of the mold and to suppress void formation. Operating tool steel dies above the phase transformation temperatures causes undesirable dimensional changes and leads to early deterioration of the dies through heatcheoking and other failure mechanisms related to cyclic thermal fatigue. This results in short die life in terms of numbers of castings.

Attempts have been made to pass Water through dies used for pressure die casting at elevated temperatures, and thus cool the dies so that they could be used satisfactorily to cast metals melting at higher temperatures than could be done otherwise. However, if the dies are operated at too low a temperature, very poor cast surfaces result from instantaneous freezing of the molten metal as it initially splashes against or flows over the cold wall. This results in surface defects in the castings known in the art as laps, cold-shuts and of other types, and, further, leads to non-uniformities in the cast grain structure and metallurgical properties of the castings. It would be most desirable to be able to operate a pressure die casting die at a suitably elevated temperature, yet with good enough thermal conductivity so that the castings freeze at a satisfactorily rapid rate. A rapid but controlled rate of freezing not only increases the operating speed of which the die casting machine is capable, but also beneficially affects the metallurgical structure and its uniformity in the casting. In addition to the other limitations of die steels, the thermal conductivity of a typical die steel such as H=13, whether water cooled or not, is too low to permit satisfactory pressure injection die casting of ferrous metals to yield smoothsurfaced, precise, uniform, fine-grained castings on a repetitive basis, as for extended production runs.

Because of the great thermal conductivity of copper, copper-lined dies must be maintained at a relatively high temperature to avoid premature cooling defects in the cast surface. Although heating and cooling means can be employed in dies which have copper faces to obtain predetermined die temperatures, an uncoated copper mold heated to 1000 F. prior to casting molten iron at 2500 F. can reach an instantaneous surface temperature of 1500 F. on casting. Even higher temperatures may be reached by the mold when other ferrous alloys such as the steels are cast at temperatures in the range of 3000 F.

At these temperatures, copper becomes very weak, and will readily flow and deform under the high pressures of pressure injection die casting. Thus, mold lives will be excessively short and uneconomical. Although copper can be alloyed, such as with beryllium or zirconium, to improve its strength at low and medium temperatures, such alloying does not impart strength high enough to be satisfactory for ferrous metal casting, and is, in a way, self-defeating, since such alloying markedly degrades the thermal conductivity of the copper, thus actually causing interface temperatures to reach higher instantaneous levels than they otherwise would.

Processes have been developed for semi-permanentmold casting of ferrous metals in dies lined with heavily anodized aluminum. However, such heavy anodization greatly diminishes the heat transfer capabilities of aluminum. It can be shown that pure, uncoated aluminum will melt in contact with molten iron at any precasting mold wall temperature if heat transfer is not impeded at the interface, such as by the anodized insulating barrier. Thus, aluminum, despite its relatively high thermal conductivity, suffers as a consequence of a relatively low melting point, and is not suitable as a mold material for repetitive ferrous casting unless heavily protected by an insulating coating. This coating greatly impedes the utility of pressure die casting for applicability to the production of ferrous metal parts. Although molten iron can be cast into a water-cooled, heavily insulated aluminum mold, such mold cannot be preheated to a desirable temperature range to obtain smooth-surface uniform-structure castings, yet maintain the high rate of heat transfer from casting to mold necessary to achieve a uniform, fine grain structure with desirable properties.

Refractory metal inserts such as core pins have been used in processes for casting metals of lower melting points than the ferrous metals such as aluminum and brass. Notwithstanding that, ferrous metal pressure die casting has not been found feasible by the prior art.

Finally, open permanent molds of refractory metals have been used at least experimentally in the casting of ferrous metal parts. The results have generally been quite unsatisfactory in that, if such molds are heated to temperatures necessary to avoid laps and cold-shuts, oxides of the refractory metal mold liner forming at the mold surface react with the molten ferrous metal at the interface to form gasses which bubble up into the ferrous metal, causing voids and other defects in the castings as well as poor surface conditions. Although this condition may be somewhat ameliorated by protective mold coatings, such coatings tend to inhibit the rapid transfer of heat from casting to mold necessary to achieve fine, uniform grain structure.

Economic factors make the die casting of ferrous metals, particularly gray cast iron, highly desirable. Pressure injection die casting is inherently an extremely economical process for producing large numbers of identical parts, such as from 1000 to 100,000 or more parts. If the pressure injection die casting of ferrous metals were feasible, large quantities of cast iron and steel products which are presently produced as sand castings or by other less efficient processes could be converted to pressure die casting at substantial savings in process costs. Moreover, gray cast iron costs only about 1 or less per cubic inch, as compared with about 25 per cubic inch for aluminum and about 35 to 5 per cubic inch for commercial zinc die casting alloys. If ferrous metal pressure die casting were feasible, many parts presently cast in zinc, aluminum or brass could be converted to gray cast iron at great savings in materials costs. However, despite these economic incentives, the prior art has not developed a feasible method for production of large quantities of ferrous metal articles by pressure injection die casting.

SUMMARY OF THE INVENTION Thus, it is an object of the present invention to provide a practical proces for repetitive, highly economical, pressure injection die casting of ferrous metals which is capable of accurately reproducing smooth die surfaces and precise die dimensions at a high production rate and through very large numbers of repetitive castings in the same die. Furthermore, an objective of the invention is to produce improved ferrous metal castings having greater strength, ductility, uniformity of structure, soundness (freedom from porosity), and other properties generally superior to those obtainable from previously known casting methods.

Briefly stated, the present invention in one of its embodiments provides a method for pressure injection die casting articles of commerce of ferrous metals containing at least 50% by weight of iron. The casting is done in dies which have inserts or liners of certain refractory metals which have specified high heat transfer characteristics and melting points and are operated at at least certain stated elevated temperatures. This results in effectively and rapidly quenching the molten metal to a predetermined and controlled mold temperature, which is high enough to prevent premature freezing that would cause surface defects in the castings.

To a certain extent, the characteristics of the method are the same as those of conventional die casting in that molten metal is caused to move rapidly into a closed permanent die and held in the die under pressure until it is frozen. The die is then opened and the solidified casting removed, and the die is next closed in preparation for cyclical repetition of the process.

Novel characteristics of the invention include providing inserts or a relatively thick layer of a refractory metal selected from the group consisting of tungsten, molybdenum, and alloys containing at least 50% of either, over essentially all the interior surfaces of the die which come in contact with the molten ferrous metal being cast. In order to obtain the desired results, the refractory metal must have a thermal diffusivity of at least about one ft. hr., a heat diffusivity of at least about 40 B.t.u./ft. F. hr. and a melting point above about 3000 F., and the layer must be thick enough, depending on the thermal transfer characteristics of its backing, to enable and to control a rapid yet not instantaneous extraction of the heat of fusion from the molten metal and to mechanically withstand the rigors of repetitive casting. It is also necessary to maintain the dies at a temperature of about 500 F.

or higher, depending on the particular ferrous alloy being cast, but below the freezing point of the ferrous metal being cast, while casting the metal, so as to essentially prevent surface irregularities and structural nonuniformities and discontinuities in the castings which would be caused by premature freezing of the liquid metal as it is being moved into the die. To the best of applicants knowledge, only the claimed refractory metals are capable of economic application in this process and demonstate adequate heat transfer capabilities at the elevated temperatures required for successful ferrous metal pressure die casting, in combination with adequate strength and durability to withstand large numbers of repetitive die casting cycles.

Thermal diffusivity and heat diffusivity are useful measurements of the rate at which heat will be extracted from a casting by a mold material. Thermal diffusivity is defined as K/ C where K is thermal conductivity in units of B.t.u./ft. F. hr., p is density in units of lb./ft. and C is heat capacity in units of B.t.u./lb. F. Thermal diffusivity is a measure of how fast heat can be transferred through a mold. Heat diffusivity is defined as the square root of the product K C and is a measure of the heat absorbing ability of a mold material. B.t.u. means British Thermal Units, lb. means pounds avoirdupois, ft. means feet, hr. means hour, and temperature is measured in degrees Fahrenheit. Percentages herein are by weight except where indicated otherwise.

The several particular embodiments of the process of the invention include casting alloys of iron and carbon containing more than the equivalent of about 1.7% carbon, and therefore containing eutectic portions on solidification, sometimes known as eutectiferous alloys, and generally known as cast irons, including white and gray cast irons; casting alloys of iron and carbon containing less than the equivalent of about 1.7% carbon, below the range of eutectic formation, and generally known as steels; casting alloys predominantly of iron, chromium, carbon, and optionally also nickel generaly known as stainless steels, both austenitic and other types, with the die lining maintained at at least about 700 F.; casting and then converting a fine-grained white cast iron to a fine-grained malleable cast iron by subsequent heat treatment in the range of about 1650 F.l850 F. for a time in the range of about one hour or more to precipitate compact nodules of graphite, decomposing essentially all the cementite present, with the ferrite grains generally smaller than the graphite nodules; and casting steels and subsequently heat treating them in the temperature range of about 1300 F.-1800 F. for a time in the range of about 15 minutes or more to refine the austenitic grain size and produce a fine-grained and strong steel article of manufacture. The invention is also useful in producing other ferrous metal alloys including low alloy and other alloy steels and ductile cast iron.

The duplex processes of pressure die casting followed by appropriate heat treatments to produce malleable iron and annealed steels result in great economy in that the heat treatments necessary to effect the desired metallurgical conversion require much less time and lower temperature than heat treatments for corresponding sand cast metals.

Processes of the invention produce articles of manufacture which have particularly unique and desirable properties in terms of strength, ductility, corrosion resistance, vibration resistance, and in other ways. Many of these unique properties are related in some way to the controlled, yet extremely rapid and drastic quench available at relatively high temperatures by die casting in heated dies of refractory metals under great pressures. Each of the specific embodiments of the process of the invention for producing gray cast iron, white cast iron, malleable cast iron, plain carbon steel, grain-refined steels, alloy steels, tool steels, stainless steels and other metals are capable of producing novel articles of manufacture having novel properties including controllable smooth surfaces and precise dimensions which accurately reproduce the surfaces and contours of the refractory metal layer which forms the inner surface of the die, uniformity of metallurgical structure at a substantial depth in the casting from all outer surfaces, physical soundness in terms of lack of substantial porosity, and fine-grain as-cast structures which greatly increase the strengths of said cast objects in comparison with the same composition cast in a sand mold.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic drawing of the central operating parts of a cold chamber pressure injection die casting machine, illustrating part of the method of the invention. In particular, it shows molten ferrous metal being poured into the chamber from which it is forced into the die for casting.

FIG. 2 shows the same apparatus after the molten metal has been forced into the die and as it is solidifying.

FIG. 3 once again shows the same apparatus, but in this case after the die has opened to permit removal of the casting and its feeding system or sprue. FIG. 3a shows the die block with a different embodiment of the invention, that is, a relatively thin layer of refractory metal lining the die in place of the massive inserts of refractory metal shown in FIGS. 1 through 3.

FIG. 4 is the iron-rich end of the iron carbon phase diagram, locating specific compositions described herein.

FIGS. 5 through 9 are sets of photomicrographs showing the effects on metallurgical microstructure of the method of the invention as compared with prior art processes including sand casting and large ingot casting. In each case, the microstructure for a particular pressure injection die cast ferrous metal alloy is shown as cast over the letter a and, in several cases, corresponding microstructure produced by sand casting and equivalent in composition is shown over the letter b. Each of the photomicrographs is originally at a stated magnification before about one-third reduction for reproduction in the printed patent. Therefore, the actual magnification of the figures as shown in the printed patent will be about 67% of the stated magnification.

FIGS. 5a and 5b respectively show die cast and sand cast gray iron having a carbon equivalent of about 4.3% at a magnification of 200x FIG. 6a shows die cast white cast iron at 500x. Sand cast white cast iron would look about the same but with somewhat larger grain and particle sizes.

FIGS. 7a and 7b at 200x respectively show malleable iron produced by annealing die cast white cast iron of the type shown in FIG. 6a and comparable sand cast white iron. The malleable iron produced from die cast white iron was produced by annealing 2 hours at 1650 F., a greatly shorter time than is necessary for commercial production of malleable iron from sand cast white iron which would normally be done for 25 hours or more at 16501850 F., even when chill plates are used to facilitate white iron production such as in thick sections.

FIG. 8a at 200x shows die cast low carbon steel having a carbon content of about 0.20%, commonly known in the AISI-SAE nomenclature system used herein as 1020 steel. FIG. at is the microstructure of the die cast 1020 steel of FIG. 8a after a heat treatment or 15 minutes at 1650 F. Although there is no FIG. 8b to illustrate the structure of 1020 steel as sand cast, FIG.8b if present would appear substantially identical to FIG. 9b. FIG. 8d at 100x shows the metallurgical microstructure of 1020 steel cast by commercial techniques in large ingots which have been wrought extensively by cold rolling and then annealed 15 minutes at 1650 F. to produce a desirable fineness of grain size for superior properties. This grain size is still larger than the annealed die cast grain size of FIG. 80.

FIGS. 9a and 9b at 200x respectively show 4618 steel as die cast and as cast in a sand mold. This low alloy steel has a nominal composition of about 1.92% nickel, 0.25% molybdenum, 0.39% manganese, 0.25% silicon, and 0.15% carbon, balance essentially all iron. FIGS. 90 and 9d, also at 200x, respectively show the microstructure of the castings of FIGS. 9a and 9b after annealing for 15 minutes at 1650 F.

FIG. 10a at 200 shows die cast austenitic stainless steel of the 18-8 type known as 304 stainless steel, having a general composition of about 18% Cr, 8% Ni, 0.08% 0., balance iron. This structure has an unusually small extent of solidification segregation. FIG. 10c, also at 200x, shows the microstructure of the metal of FIG. 10a after annealing for one hour at 1950 F.

FIGS. 11a and 11b are comparative illustrations of the metallurgical surface conditions of gray cast iron castings produced in refractory metal mold inserts by pressure die casting. Both castings are about 3 x 1 /2 x /2 inches large, except for flash around parts of the edges. FIG. 11a shows typical surfaces produced with the mold at about 350 F. from heating the outsides of the dies with torches, while FIG. 11b shows the greatly improved surfaces produced by preheating the mold to a temperature of about 700 F. before casting.

DESCRIPTION OF THE PREFERRED EMBODIMENTS [In our efforts to develop and perfect the present invention, we have proven feasibility of the process and substantial, improvements in product properties by pressure injection die casting in refractory metal mold inserts many parts of gray cast iron, white cast iron, 1020 low carbon steel, 4618 low alloy steel, and 188 stainless steel, and also have produced greatly improved parts of malleable iron and annealed low carbon steel. It is expected that similar improvements could be obtained in ductile cast iron and other ferrous alloys.

The use of refractory metal die inserts maintained at elevated temperatures above 500 F. has made possible these successes. Table I below compares the thermal diffusivity and heat diffusivity of tungsten and molybdenum with a typical tool steel used for dies, H-11 die steel, similar to H-13 and other commercial die steels. The thermal diffusivity and heat diffusivity parameters were defined above in the summary section.

TABLE I.HEAT TRANSFER CHARACTERISTICS Various alloys of tungsten, molybdenum, or both together are also suitable as die inserts in accordance with the invention so long as the thermal diffusivity is greater than about one ft. /hr., the heat diffusivity is greater than about 40 B.t.u./ft. F. hr. and the melting point is above about 3000" F. Although the melting point of the refractory metal alloy itself should be above about 3000 F., certain composite materials could be satisfactory having small amounts of lower melting materials dispersed through the refractory metal, such as copper infiltrated tungsten, so long as there is not even incipient melting of separate minor phases at so low a temperature as to make the alloy unsuitable for reasons such as weakness. The thermal diffusivity and heat diffusivity depend on thermal conductivity, density and heat capacity and determine the rate at which castings can be produced in die inserts or composite cavities made in acordance with the present invention and the type of effective solidification and quench given to the castings. For practical application, it is desirable that the refractory metal die liners of the invention be repairable by building up material thickness by means such as welding, plasma spraying, drilling substantially higher than that of the metal being cast, to

effect a very rapid and nearly uniform solidification of the molten metal to produce a precisely shaped, useful article of solid, cast metal. Such articles are characterized by smooth surfaces, an unusual degree of fineness of grain structure, and superior physical properties such as tensile and rupture strength, ductility, heat-treatability, and, in some cases, corrosion resistance. Preferably, the enti remass of molten metal is injected into the heated die in a very short time, which can be less than about one second depending on the size of the mold. With most ferrous metal alloys being cast, solidification can be completed in processes of the invention within less than about two seconds if the maximum section thickness of the casting is less than about /2 inch. For greater section thicknesses, very rapid solidification of the outer layers is still achieved, producing thick, uniformly fine-grained surface layers, depending on the metal, from about a inch to a /2 inch thick. Preferably, turbulence of the liquid metal being injected throught a restricted gate at high pressures, followed by immediate application of the full design pressure of the die casting machine permits the high heat transfer properties of the die lining to rapidly extract heat from the liquid metal.

In contrast with pressure die casting practices of the prior art where relatively high heat conductivity materials are cast in dies which have low heat conductivity, the present invention provides for the casting of metals of low heat conductivity in dies Which have high heat conductivity. Therefore, beyond a certain point, the rate of heat transfer from casting to the mold during freezing will be determined primarily by the rate of transfer of heat energy from the interior of the casting through the frozen metal to the mold wall surface rather than the temperature differential at the interface between the mold and the casting. The versatility of the present inveniton is further demonstrated by the realization that the ability to keep parts of the mold at quite high temperatures permits controlled uniform freezing of complex casting configurations to eliminate thermal stresses in the solidified metal which might otherwise be caused by nonuniform or zonal freezing and cooling. As is known in the art, water cooling and insulating back-up layers can be used to control different rates of heat removal from different parts of the castings, as desired.

To get smooth surfaces and uniform structures in the castings, the mold surfaces are kept above about 500 F., preferably, in the range of about 7001000 F., depending on the particular metal being cast, maintaining a high degree of heat fiow from the casting to and through the mold. This means that the use of coatings to insulate mold surfaces is not desirable; however, a thin wash coating to prevent adherence or soldering of the casting to the mold and to facilitate casting removal is permissible as long as it does not substantially restrict heat transfer.

Turning now to the drawings, FIGS. 1 through 3 illustrate the process of the invention. In FIG. 1, a conventional die casting machine, preferably of the coldchamber type, is illustrated at 1 by a box in dashed lines. A cold-chamber die casting machine is as distinguished from a hot-chamber machine. In a cold-chamber machine the molten metal 5 is transferred manually as by pouring from ladle 2 through metal supply opening 15,

or automatically, to a shot-sleeve 3. Plunger 4 is designed to push the molten metal 5 from shot-sleeve 3 through gate 16 into die cavity chamber 6 when plunger 4 is moved by an external power source as indicated at 7, such as a hydraulic cylinder. By contrast, a hotchamber machine, which could be used with the present invention, provides for automatic pumping of the molten metal from beneath the surface of a holding tank of the metal, not shown in these drawings. Thus, the pressure source or pump in a hot-chamber machine is normally immersed in the molten metal and operates at the temperature of the molten metal. It will be understood by those skilled in the art that a hot-chamber machine could be made to operate in the pressure die casting of ferrous metals if the metal pump was suitably designed of suitable materials.

In the present invention, the die comprises a movable half 8 and a fixed half 9. Massive inserts of refractory metals are illustrated at 10 and 11 and are fixed in each of the halves 8 and 9. Suitable means are provided for moving the movable half of the die 8 with is refractory metal insert 10 away from the fixed half 9 and its insert 11, such as by means of toggle linkages 12 and 13. Suitable die casting machines known in the art provide substantial restraint between die halves 8 and 9 and in relation to the energy source 7 for plunger 4, so that the dies will not be forced open by the very large pressures generated in the liquid metal casting by plunger 4. Separating forces tending to force the dies open can be quite large, depending on the pressure used and the projected area of the casting. In the present invention, pressures preferably in the order of about 10,00030,000 pounds per square inch are generally used, although substantially lower pressures may be used within the scope of the inventiou.

FIG. 2 illustrates the die casting machine of FIG. 1 in which plunger 4 has forced liquid metal 5 into the casting cavity 6. Excess molten metal is present in the feeding system 14 which includes biscuit 18, and gate 16. In accordance with the invention, the liquid metal 5 is preferably caused to move into the cavity 6 very rapidly, such as in considerably less than one second, and would normally freeze substantially completely within less than one or two seconds after it fills the mold. As can be seen in FIG. 2, the plunger 4 seals off the metal supply opening 15 as plunger 4 advances past opening '15, so that molten metal 5 is forced into die cavity 6.

As stated elsewhere in this application, it is essential in the present invention that the refractory metal inserts be maintained at temperatures high enough to prevent premature freezing of the cast metal to avoid casting surface defects and faithfully reproduce the die surfaces, generally above about 500 F., the actual temperature depending on the metal being cast. Such temperatures can be maintained by the use of electric heaters 19 in the die block itself, by preheating the die blocks internally or externally with torches or otherwise before commencing casting operations, and by other means. With one set of dies used by applicants, 4 kilowatts of electric heat input was more than sufiicient to raise the dies to and maintain them at about 700 F. operating temperature. Once casting operations are commenced, the casting operation itself tends to keep the dies at quite high elevated temperatures, and supplementary heat input may or may not be necessary, depending on the temperature of the metal, the time of the casting cycle, the heat absorbing ability of the die casting machine and its environment, and other factors.

FIG. 3 illustrates the same die casting machine after the casting has solidified. Toggles 12 and 13 have opened to pull the movable die half 8 with its refractory metal insert 10 away from fixed die half 9 With its refractory metal insert 11. Simple means such as knock-out pins known in the art are normally provided to remove casting 17 with its solidified biscuit 18 from the movable half of the die 9 once the movable half of the die 8 has moved out of the way. After removal of casting 17, the biscuit 18 and gate 16 can be cut off at section xx. Plunger 4 has retracted beyond the metal inlet 15 to allow metal to be poured in for the next casting. Also, the dies can be closed again as illustrated in FIG. 1 to prepare for the next casting cycle.

FIG. 3a illustrates another embodiment of a die made for use with the invention having a relatively thin layer of refractory metal liner in the die rather than a more massive insert.

Although sometimes preferably, it is not necessary that refractory metal inserts as massive as those illustrated at 10 and 11 be used. The necessary minimum thickness of the refractory metal layer will depend on the back-up material, as well as the nature of the ferrous alloy to be cast. With a back-up material that has high heat conductivity, thinner refractory metal layers can be used, depending also on the respective thermal expansion characteristics of the back-up and liner materials. Also, composite dies made of several layers of different materials are conceivable, so long as the material facing the molten metal is a refractory metal of the invention and is thick enough to control the heat transfer characteristics of the mold and withstand the rigors of repetitive casting. Preferably, massive inserts of at least /2 inch minimum thickness of refractory metals are used in die steel molds. However, thinner inserts which might be made by plasma spraying suitable refractory metals on other materials such as copper alloys or die steels could have a refractory metal thickness as little as about 0.06 inches or less.

A preferred refractory metal is unalloyed molybdenum produced by conventional commercial powder metallurgical techniques. Unalloyed tungsten, also preferably produced by powder metallurgical techniques, or produced by are melting or electron beam melting, is also desirable. For increased strengths at higher temperatures, generally with some sacrifice in heat transfer characteristics, alloys such as molybdenum 20% tungsten are useful as mold lining materials. Specifically, an alloy of 79.8% molybdenum, 20% tungsten, 0.2% thoria, described and claimed in Pat. 3,285,736Brinn and Barto, assigned to the assignee of the present invention, is suitable. Other molybdenum and tungsten metals, Wrought tungsten, wrought molybdenum, and alloys that have suitable thermal characteristics can be used, including tungsten with about 2% thoria and, in some applications, copper-infiltrated pressed and sintered tungsten. Of course, if the refractory metal part of the die is thick enough, it can be used Without die steel or other back-up material.

The rapid cooling effects of refractory metal molds largely determine the structure of ferrous metal castings made in accordance with the present invention. These effects control the entire structure of thin castings, and they cause quite thick outer layers of fine-grained structure in thicker castings, with the thickness of the outer layer depending on the metal. It can be at least as thick as inch in gray iron, thinner in stainless steels which have poorer heat conductivity. To take full advantage of the invention, the working portions or highly stressed portions of castings should be thin enough to have structures and grain sizes characteristic of the invention throughout their cross sections. Thus, the cross sections in these portions should not be so thick that the preferred structure does not extend substantially through it. Other portions of the same casting which are not intended to be so highly stressed in use can be made thicker and have a cored structure with more conventional structure and grain sizes at the center.

To meet the requirements of the invention, the mold or die, considered as a structural entity, must have certain minimum properties of strength, thermal absorptivity and conductivity, and melting point. To resist plastic deformation at the maximum temperatures and pressures reached in pressure injection die casting of ferrous alloys, the yield strength of the die material, whether in a monolithic block or within a composite or layered structure, such as for example a layer of molybdenum on a backing of alloy steel or copper-beryllium alloy, must be such at every point within the structure so as to resist the stresses at that point as determined by injection pressure, cavity geometry, and distance of said point from the mold-casting interface. Furthermore, it must be capable of resisting said stresses at the maximum temperatures reached at that point during repetitive casting of high melting ferrous alloys. Further, the thermal absorptivity, which is the heat diffusivity, of the mold structure must be such that the heat of fusion together with any usual degree of superheat can be removed from the critical section of the casting with sufficient rapidity to achieve unusual fine-grained cast structure. Also, it is necessary that the thermal conductivity of the mold structure be sufficient so that such heat of fusion, superheat, and any portion of residual heat transferred to the mold following solidification but prior to ejection of the casting, be transferred by the mold to the heat sink, i.e., into cooling water, conducted to the body of the casting machine, or radiated to the atmosphere, or otherwise disposed of, with sufficient rapidity so that the average temperature of the mold surface preferably remains essentially constant with time during extended sequences of repetitive casting, or at least does not reach deleteriously high levels. The average operating temperature is determined from the maximum and minimum temperature occurring during cyclical operation. Further, it is highly desirable that the mold surface have a melting point substantially higher than that of the metal being cast in order that it better may resist wetting and erosion by the liquid metal and, most importantly, that such mold may be heated to substantially elevated temperatures prior to casting so as to avoid premature surface freezing of the casting, yet without endangering the finish and integrity of the mold surface as a result of the higher instantaneous surface temperatures experienced during contact with injected liquid metal.

In brief, to meet the requirements of this invention, the properties of the mold must be such that it not only has a high degree of permanence for extended casting operations, but also such that the precise dimensions and surface finish of the casting produced therein are determined by the internal, heated surfaces of the mold itself rather than by a prematurely frozen, often imperfect, skin of metal instantaneously frozen at cold mold surfaces.

It will be apparent that the minimum properties which a mold must have to meet the above requirements will depend on several parameters including: the size and shape of the casting, the precasting mold temperature, the temperature at which the liquid metal is injected into the mold, the pressure of injection, the heat capacity, density, and thermal conductivity of the particular metal or alloy being cast, and other factors. It also will be apparent that different combinations of refractory metal mold surface and substrate can be devised to meet such requirements, depending on all the above parameters as well as the particular mold material, back-up material, heat-sink mechanism, and other parameters.

The invention is applicable to the production of a broad variety of articles of commerce cast from ferrous metals containing at least 50% iron. By way of brief example, and not limiting the scope of the invention, the invention can be used to produce such products as: automotive components such as rocker arms, steering knuckles, bearings, and fittings; appliance parts such as linkages, gears, valves, and pulleys; aerospace parts such as turbine blades; architectural fittings; miscellaneous hard- Ware; and many other types of products.

Iron-carbon diagram FIG. 4 is the iron-rich end of the iron-carbon phase diagram. It will be used here to discuss aspects of the invention requiring an understanding of ferrous metallurgy. The characteristics of the iron-carbon diagram are well known in the art and are rather complex. Therefore, they will not be discussed in detail here, but they will be discussed to an extent desirable to aid metallurgists skilled in the art in understanding the phenomena of the present invention.

The phase diagram shows the phases present at quasiequilibrium at the indicated temperatures with the indicated percentages of carbon in a binary iron-carbon alloy. Although often referred to as an equilibrium diagram, it actually is not in that Fe C, called iron carbide or cementite, is thermodynamically unstable over a wide range of temperature with respect to decomposition to carbon and iron. Upon addition of other alloying elements such as chromium, nickel, phosphorus, and silicon, various shifts in the diagram will occur. To describe the diagram in general terms, pure iron containing no carbon is seen to melt at about 2800 F. The minimum melting point for the eutectic composition occurs with 4.3% carbon at 2066 F. With other alloying additions, commercial gray cast irons can be found to melt at lower temperatures.

Ferrite (a-iron) and fi-iron have body-centered-cubic (BCC) crystalline structures, while austenite ('y-iron) has a face-centered-cubic (FCC) structure. Austenite, ferrite and 6-iron are solutions of carbon in iron. Although austenite normally is not stable at temperatures below about 1330 F., its stability at lower temperatures may be enhanced by certain alloying additions, such as nickel, for example, and steels with sufficient alloy content to stabilize austenite to normal temperatures are called austentic steels.

Iron-carbon alloys containing more than about 1.7% carbon or the equivalent thereof, which is the maximum amount of carbon soluble in the composition are known as cast irons, and are characterized by various amounts of eutectic, often lamellar, structure containing 4.3% carbon and forming on final solidification. This is known as ledeburite. Ironcarbon alloys with less than 1.7% of carbon are known as steels and are characterized by various amounts of lamellar eutectoid structure on cooling through the final transformation of austenite at 0.80% carbon and 1333 F. This is known as pearlite. Steels having more than the equivalent of 0.80% carbon are known as high carbon steels. Those having less are known as low carbon steels.

The morphology of the ferrite, cementite and graphite in iron-carbon alloys is most important in determining the strength and other properties of the alloys. Ironcarbon alloys, especially the steels, are quite sensitive to heat treatments which cause variations in their structure and can harden or soften the metal in various ways.

Generally, cast steels must be annealed above line A in FIG. 4 to refine the size of the austenite grain boundaries residual in the structure and to give a finer, stronger and more useful structure. It can be seen that this would require that a 1020 steel, which contains about 0.20% carbon, :be heated above about 1560 F. for grain refinement. As pointed out in the following examples, 1020 steel cast in accordance with the present invention seems to transform to austenite to a much greater extent than could be expected at temperatures substantially below 1560 F. leading to satisfactory grain refinement. Although this cannot be explained or defined in directly measurable properties of the metal, the castings can be defined by the process used to produce them. This phenomenon can be of substantial economic importance.

The cast irons often contain silicon and other alloying elements. The carbon equivalence of a cast iron is determined by adding to the actual percentage carbon content one-third of the silicon percentage and making other adjustments known in the art for other elements present such as phosphorus. With high carbon equivalence levels, such as over about 4%, the solidified product is gray cast iron, which is a matrix of ferrite with dispersed graphite. If any pearlite is present, proper heat treatment will convert it to ferrite and graphite. 1f higher strength is de sired, the cast-ing can be annealed but only at lower temperatures to remove casting stresses leaving the metal matrix in a pearlitic condition. Carbon equivalents over 4.3% are generally undesirable since coarse graphite flakes, known as kish, form in the melt and can deleteriously affect the castings. Graphite flakes in cast iron can be considered mechanically as almost a notch or void, so the size, shape and distribution of graphite in cast irons is most important.

Because of the greatly lowered melting point, cast irons are preferable to steels for many applications. Also, solidification shrinkage of the castings on cooling is at a minimum for most gray cast irons, thus facilitating the casting of complex parts. However, cast irons are notoriously weaker and more brittle than steels because of the graphite in the structure.

To obtain greater ductility in cast irons, malleable iron or ductile iron can be used. Malleable iron is produced by annealing white cast iron (a metastable structure containing no free carbon) to cause the graphite to form as relatively compact nodules, similar in shape to popcorn balls, in contrast to the elongate flake graphite types in gray cast iron. The white cast iron from which malleable iron is prouced is a low carbon cast iron, such as containing 2.5% carbon and 1.5% silicon for a carbon equivalent of 3.0%. White iron can be produced with lower carbon contents in sand molds and with relatively high carbon contents in chill molds. White iron is cast iron containing ferrite and cementite, with much pearlite, and no free graphite. It is quite strong, hard and brittle. On proper annealing, the cementite decomposes to give graphite nodules in ferrite, or, if preferred, in a pearlitic matrix. This is malleable iron and is much more ductile and softer than white iron.

Graphitizing agents such as silicon, nickel and copper encourage the formation of gray iron rather than white iron. Gray iron metal articles of the invention tend to form with a white iron case or surface around a gray iron core. This case can be converted to malleable iron by appropriate heat treatment, and graphitizing agents will minimize thickness of the white iron case.

As is known in the art, ductile iron with quite spherical graphite particles can be produced by adding certain innoculants to the melt just before pouring the castings. We have no reason to believe that the present invention would not be fuly applicable to the production of improved ductile iron castings.

Introduction to specific examples Although the beneficial effects of the invention still cannot be explained fully, even in hindsight, the following thoughts and hypotheses will aid the reader in understanding to a certain extent some mechanisms which may contribute to the greatly improved properties of articles of commerce made according to the invention, which properties are more fully disclosed in the specific examples.

From an examination of the metallurgical structures of ferrous articles pressure injection die cast in refractory metal molds, such as those structures depicted in FIGS. 5a, 6a, 8a, 9a and 10a, it will be apparent to those skilled in the metallurigical arts that the structures of said metallic articles are characterized by an unusual degree of fineness of grain, in some cases more than an order of magnitude smaller in average unit grain area than normally is observed for metal of similar composition cast by conventional techniques, such as by sand mold casting as depicted in FIGS. 5b, 7b and 9b. A certain degree of fineness of grain in these die cast ferrous structures was not entirely unexpected, since a relatively high rate of heat removal from the castings by the refractory metal mold material was anticipated. The metallurigical art teaches that grain size in cast metal structures is strongly influenced by cooling rate, and that faster cooling rates yield finer grains than are observed with slow cooling rates, which normally lead to coarse grain structures. However, it should be realized that smooth-surfaced, extended bodies of ferrous alloy metals having such uniform, exceedingly fine-grained, as-cast structures have not been known heretofore, owing to the great difficulty in achieving rapid removal of the heat of fusion and superheat from a liquid ferrous metal through the surface of a conventional mold material, yet with the avoidance of premeature chilling and freezing leading to surface defects and casting imperfections. Further, it was entirely unexpected that, in metal sections up to at least /2 inch in thickness, a remarkable uniformity of such exceedingly fine cast grain structures across the entire thickness of the section would occur in die cast ferrous alloys, such as low carbon and alloy steels for example. Also, the usual type of nucleation and grain growth progressing slowly inward from the surface, which leads to an undesirable dendritic type of cast grain structure, ap pears not to be operative in the solidification of such cast sections except only with certain ferrous alloys having very poor thermal conductivity, such as certain highly alloyed stainless steel compositions; however, even such highly alloyed compositions are characterized by an unusual degree of fineness of dendritic-type grain structure and a uniformity of average grain size across cast sections up to at least one-half inch in thickness.

It is difficult to explain this unusual uniformity of exceedingly fine grain structure on the basis of hitherto known metallurgical practices. We believe that the very rapid injection of the mass of molten metal through the narrow gating system into a preheated cavity mold of high thermal conductive and absorptive properties may allow a supercooling of the entire mass of the casting to a remarkable degree immediately prior to solidification, yet without premature freezing at mold wall surfaces, said degree of supercooling being further accentuated by the application of pressure during the freezing process, so that the supercooled molten metal nucleates and solidifies nearly uniformly and instantaneously once solidification commences rather than the usual process of solidification common to most casting practices in which the metal slowly and sequentially freezes from the mold wall interface into the center of the casting. The normal freezing process leads, of course, to difierences, sometimes drastic, in grain structure from edge to center in ferrous castings, to undesirable segregation of impurities as well as intentionally added alloying agents from point to point within ferrous castings, and to inherently weak, dendritic grain structures.

In addition to the important benefits of uniform fine grain structure to the strength and ductility of cast ferrous metals, certain unexpected ancillary benefits are realized as a consequence of these unusual structures obtainable in pressure injection die casting of ferrous metals in refractory metal molds. Owing to these exceedingly fine-grained structures, desirable alterations in metallurgical structure to be produced by thermal treatments, such as the conversion of cast white iron to a nodular graphite dispersion in a ductile ferrite matrix to produce malleable iron, and the annealing of alloy steel to improve ductility and other properties can be accomplished in significantly shorter times and at somewhat lower temperatures than are conventionally employed in the ferrous metallurgical art and with retention of a desirable, uniform fine grain size as illustrated in FIGS. 7a, 80, 9c and 100. We believe such phenomena may be due in part to the relatively short diffusion distances necessary for structural transformations to occur in exceedingly finegrained structures, together with the somewhat higher free interfacial energies of such structures. Further, in the alloy and stainless steels, the combination of ultra-fine grain size together with the repression of segregation of impurities at grain boundaries during the rapid solidification process would significantly improve the resistance of such materials to corrosion and attack by chemicals and certain other reagents.

To aid further in understanding the specific examples, the carbon equivalent of several of the examples is located on the phase diagram of FIG. 4.

SPECIFIC EXAMPLES Example 1Gray cast iron A ferrous metal alloy of the composition: 3.5% carbon, 2.2% silicon, 0.7% manganese together with incidental amounts of other elements such as phosphorus, sulfur and others, balance iron, a member of the class of alloys known generally as gray cast iron and designated specifically as having a carbon equivalent of 4.23%, was melted in an electric induction furnace. A suitable amount of this metal in the molten condition was ladled at a temperature of about 2400 F. into the injection chamber of a pressure injection die casting machine, whereupon it immediately was caused to flow under the action of a plunger moving at a rate of 35 feet per minute (ft/min.) and to be injected under a pressure of 3000 pounds per square inch (p.s.i.) through a narrow gate approximately 0.125 x0.500 inches into a shaped cavity previously formed within adjoining die insert blocks of pressed and sintered molybdenum fitted into a back-up steel mold block, said said process being as illustrated in FIGS. 1 and 2. Prior to injection of the molten ferrous alloy, the mold surfaces were maintained at a temperature of about 500 F., in part by internal, controllable, electric heaters built within the mold structure behind the mold surfaces and in part by residual heat from previous castings made in the same mold. Within a period of less than about two seconds, the entire mass of injected molten metal had solidified into a solid body reproducing precisely the shape and closely duplicating the surface of the pressed and sintered molybdenum mold cavity, whereupon the adjoining halves of the mold cavity were caused to open by the mechanism of the casting machine and the solid metal part was objected as illustrated by FIG. 3. The unusual as-cast grain structure of the solidified metal part is illustrated in FIG. a. Mechanical properties of the cast metal after annealing at 1650 F. for about 2 hours are documented in Table II in comparison with properties for metal of the same composition cast in sand molds and given the same anneal.

TABLE II.GRAY IRON PROPERTIES In the tables, k.s.i. means thousands of pounds per square inch, and R means hardness on the Rockwell B scale. Rupture tests were made by three-point bending of bars 3 x 1 /2 x /2 inches with 2 inches between supports. All tests were made at room temperature of about 77 F. Tensile tests were mostly performed on standard machined button head specimens with a gauge diameter of 0.250 inches and length of about 1.3 inches, using an elastic strain rate of 0.005 inches/inch/ minute (in./in./ min.) and a plastic strain rate of 0.05 in./in./min.

The annealing treatment results in a maximum soft condition so as to allow equal comparison of properties without regard to strength improvements that may be obtained by other known heat treatments. Also, all hard constituents such as massive carbides, which are present on the surfaces of the cast articles are decomposed to graphite and ferrite by this heat treatment.

The finer graphite flake size and more uniform distribution of the flakes, as compared to the sand cast article (FIG. 5b) of equivalent chemical composition, have re- 16 sulted in higher tensile strengths and higher rupture stress values for the pressure injected die cast article.

Example 2Malleable cast iron A ferrous metal alloy of the composition: 2.5% carbon, 1.5% silicon, 0.45% manganese, 0.6% molybdenum, together with incidental amounts of other elements such as phosphorus, sulphur and others, balance iron, to be cast as white iron, having a carbon equivalent of 3.0% and subsequently to be annealed to form malleable cast iron, was melted in an electric induction furnace. A suitable amount of this metal in the molten condition was ladled at a temperature of about 2450" F. into the injection chamber of a pressure injection die casting machine, whereupon it was cast in the same manner as Example 1. The as-cast grain structure of the solidified white cast iron metal part is illustrated in FIG. 6a and the corresponding malleable iron structure after a thermal annealing treatment of 2 hours at 1650 F. are illustrated in FIG. 7a. Mechanical properties of the annealed cast metal are documented in Table III in comparison with properties for metal otherwise the same but which was cast in said molds.

TAB LE III.-MALLEAB LE IRON The as die cast white cast iron structure is rather typical for the chemical composition described. With the exception of the fineness of the cells, only minor differences can be detected between die cast and sand cast white cast iron structures. Of significant importance however is the ease with which the white cast iron can be converted to a malleable iron. Heat treatments as short as one-tenth as long as commercially accepted can be used to obtain the structures shown in FIG. 7a. The fineness of the die cast malleable iron structure shown in FIG. 7a as compared to a sand cast malleable iron structure, as shown in FIG. 7b, also contributes to higher strengths and hardnesses. Optimization of chemistry and treatments would lead to even further improved properties.

Example 3-1020 low carbon steel A ferrous metal alloy of the composition: 0.18% carbon, 0.12% silicon, 0.25% manganese, together with incidental amounts of other elements such as phosphorus, sulfur, aluminum and others, balance iron, a member of the class of alloys known generally as low carbon steel, and designated specifically as AISI 1020 steel, was melted in an electric induction furnace. A suitable amount of this metal in the molten condition was ladled at a temperature of about 2950 F. into the injection chamber of a pressure injection die casting machine, whereupon it was cast in the same manner as Example 1. The significantly different as-cast grain structure of the solidified die cast metal part is illustrated in FIG. 8a, and the corresponding structure after a thermal annealing treatment of 15 minutes at 1650 F. is illustrated in FIG. 8c. Mechanical properties of the cast metal are documented in Table IV in comparison with properties for metal otherwise the same but which was cast in sand molds.

TABLE IV.-1020 LOW CARBON STEEL The as-cast die cast low carbon steel articles show a remarkable fineness of structure and have the same general appearance as steels of eutectoid composition, 0.8% C

17 identified on FIG. 4 as Ex. 3'; even though the nominal chemical composition of about 0.2% carbon equivalent is such that one would not expect to observe such behavior. Although the literature suggests rapid cooling will shift the eutectoid structure somewhat towards lower carbon content, preliminary observations indicate considerably greater effect than we have heretofore observed in prior art practice or in the literature.

As with the unusual as-cast structure, the annealed die cast low carbon steel article exhibits extreme fineness of structure. Response to subsequent heat treatment of the die cast articles is very rapid and results in uniform finegrained structures which compare favorably to those in worked low carbon steel articles as shown in FIG. 8a.

The mechanical properties of the die cast and annealed low carbon steel articles exceed those of sand cast and annealed articles probably at least in part because of the fineness and uniformity of the structure.

Example 44618 mild alloy steel A ferrous metal alloy of the composition: 0.15% carbon, 0.25% silicon, 0.39% manganese, 1.92% nickel, 0.25% molybdenum, together with incidental amounts of other elements such as phosphorus, sulfur and others, balance iron, a member of the class of alloys known generally as nickel-molybdenum steels and designated specifically as AISI-SAE 4618 steel, was melted in an electric induction furnace. A suitable amount of this metal in the molten condition was ladled at a temperature of about 2950 F. into the injection chamber of a pressure injection die casting machine, whereupon it was cast in the same manner as Example 1. The as-cast grain structure of the die cast metal article illustrated in FIG. 9a, and the corresponding structure after a thermal annealing treatment of minutes at 1650 F. is illustrated in FIG. 9c. Mechanical properties of the cast metal and the annealed metal are documented in Table V in comparison with properties for annealed metal otherwise the same but which was cast in sand molds.

The particular time-temperature-phase transformation cooling behavior of the 4618 type of steel has resulted in microstructure difference, as compared to sand cast alloy of the same composition shown in FIG. 9b, which behavior is similar to that of 1020 steel cast as in Example 3. The shift in eutectoid composition to much lower effective carbon content is evident along with substantially finer and more uniform grain size and phase distribution.

After identical annealing treatments, there is a distinctly greater fineness and uniformity of distribution of the various phases in the pressure die cast article as compared to the sand cast and annealed article, as seen in FIGS. 9c and 9d.

As shown in Table V the strength properties of the die cast article, both as cast and annealed, are substantially superior to the sand cast article. The somewhat lower ductility in the die cast articles is not fully understood at this time, but the much higher strength, approximately 80% higher, can be expected to detract from the ductility properties of the article. To those skilled in the art, it is apparent that various combinations of strength and ducility can be obtained through the use of various heat treating procedures, and greater ductility can be achieved in the die cast product with some loss of its high strength.

1 8 Example 5-304 stainless steel A ferrous metal alloy of the composition: 0.08% carbon, 1.0% silicon, 1.5% manganese, 10% nickel, 19% chromium, together with incidental amounts of other elements such as phosphorus, sulfur and others, balance iron, a member of the class of alloys known generally as stainless steel, and designated specifically as 304 stainless steel, was melted in an electric induction furnace. A suitable amount of this metal in the molten condition was ladled at a temperature of about 2850 F. into the injection chamber of a pressure injection die casting machine, whereupon it was cast in the same manner as Example 1. The as-cast grain structure of the solidified metal part is illustrated in FIG. 10a, and the corresponding structure after a thermal annealing treatment of 2 hours at 1950 F. is illustrated in FIG. 100. Mechanical properties of the cast metal are documented in Table VI in comparison with properties for annealed metal otherwise the same but which was cast in sand molds.

TABLE VI.304 STAINLESS STEEL Unlike the other iron-base alloys previously described in Examples 1 through 4, the alloy designated as 304 stainless steel does not undergo any solid state phase transformations as it cools to room temperature. As a result of such behavior, the microstructure as shown in FIG. 10a is exactly the as-solidified austenitic microstructure of the material. Again the fine and relatively uniform as-cast microstructure is evident as previously discussed for the other alloys.

Because of the high alloy content of this material, a chemical micro-segregation occurs upon freezing the liquid alloy, but this effect is minimized by the pressure injection die casting process as compared to other casting methods. Also, relatively rapid homogenization can be accomplished with die cast articles of this composition at least partly because of the fineness of the structure as compared to sand cast articles of the same chemical composition.

The mechanical properties of the die cast 304 stainless steel articles as shown in Table VI are at least equivalent to those produced by sand casting methods and in thinner sections are observed to be substantially improved. To those skilled in the art, further improvements can be expected with respect to various mechanical and physical properties from the use of various chemical and heat treating variations.

Example 6-Gray cast iron Gray cast iron was cast successfully as in Example 1 but in wrought molybden-um die inserts using a plunger speed of ft./ min. and a pressure of 3000 p.s.i.

Example 7-Malleable cast iron Malleabel cast iron was produced successfully as in Example 2 but in copper-infiltrated tungsten die inserts using a plunger speed of 130 ft./min. and a pressure of 9000 p.s.i. Examples 2 and 7 show that white cast iron can be produced by use of the invention with unusually high carbon equivalents. Broader ranges of composition can be used than with methods of the prior art.

Example 81020 Low carbon steel 1020 low carbon steel was produced succesfully as in Example 3 but in 2% thoriated tungsten die inserts using a plunger speed of 130 ft./min. and a pressure of 3000 p.s.1.

Example 9-4618 Mild alloy steel 4618 mild alloy steel was produced successfully as in Example 4 but in wrought molybdenum die inserts using a plunger speed of 130 ft./min. and a pressure of 3000 p.s.1.

Example 10304 Stainless steel 304 stainless steel was produced successfully as in Example 5 but in wrought molybdenum die inserts using a plunger speed of 130 ft./min. and a pressure of 3000 In Examples 6 through 10 the unusual microstructure observed, as described in Examples 1 through 5, are duplicated to the extent that no differences can be detected readily by a trained observer.

As stated and documented in several places above, the novel products of the present invention cannot be adequately delineated or defined by direct statements of their chemistry, metallurgical structure or mechanical properties. In several instances, improved properties from heat treatments after die casting are major economic advantages of these products. Therefore, they are herein defined by the process found to produce them in addition to the characteristics of the products themselves.

The foregoing is a description of illustrative embodiments of the invention, and it is applicants intention in the appended claims to cover all forms which fall within the scope of the invention.

What is claimed is:

1. A process for repetitive pressure injection die casting of articles of ferrous metal containing at least 50% by weight of iron and less than the maximum amount of carbon that is soluble in the composition, comprising the sequential steps of: (A) rapidly injecting said ferrous metal in a molten form into a closed die through a gate so that the molten metal nucleates and solidifies nearly uniformly in said die, (B) holding said ferrous metal within said die under pressure while heat is extracted until said ferrous metal is substantially solidified, and (C) opening said die and removing the solidified ferrous casting from said die, and then closing said die in preparation for cyclical repetition of the process, wherein:

said die is constructed so that all of its interior surfaces which come in contact with said molten ferrous metal are refractory metal selected from the group consisting of tungsten, molybdenum and alloys containing at least 50% by weight of one or more of tungsten and molybdenum, said refractory metal having at its average operating temperature a thermal diffusivity of at least about one ft. /hr., a heat diffusivity of at least about 40 b.t.u./ft. F. hr. and a melting point above about 3000 F., said refractory metal being sufficient in thickness to enable effective control of the rate of heat extraction from said molten ferrous metal and adequate to mechanically withstand the rigors of repetitive pressure injection die casting, and

the interior surfaces of said dies are maintained at an average operating temperature in the range of about 500 to 1000 F. while casting said ferrous metal in said closed die, so as to essentially prevent surface irregularities in the castings due to premature freezing, while still allowing heat to be extracted from said ferrous metal at a rapid rate due to the thermal characteristics of said refractory metal.

2. A method of claim 1 in which said refractory metal is selected from the group consisting of tungsten and alloys containing at least 50% by Weight of tungsten.

3. A method of claim 1 in which said refractory metal is selected from the group consisting of molybdenum and alloys containing at least 50% by weight of molybdenum.

4. A method of claim 1 for casting alloys of iron and 20 carbon containing less than about 1.7% carbon in which the temperature of the inner faces of the dies is at least about 700 F. immediately prior to injection of molten ferrous metal.

5. A method of claim 1 for casting an alloy predominantly of iron, chromium, nickel and carbon in which the temperature of the inner faces of the dies is at least about 600 F. immediately prior to injection of molten metal of such composition.

6. A method of claim 4 in which said casting, after it has been removed from said die, is heated to a temperature of at least about 1,300 F. for a time of about 15 minutes or more to define the austenitic grain size and porduce a fine-grained and strong steel article of manufacture.

7. A steel article of manufacture made according to the process of claim 4 characterized by smooth surfaces which accurately reproduce the surfaces of the refractory metal which forms the inner surface of the die, uniformity of metallurgical structure a substantial distance into said casting from all outer surfaces, physical soundness in terms of lack of substantial porosity, and fine-grain as-cast structure which materially increases the strength of said article in comparison with articles of the same composition cast in sand molds.

8. A stainless steel article of manufacture made according to the process of claim 5 characterized by smooth surfaces which accurately reproduce the surfaces of the refractory metal which forms the inner surface of the die, uniformity of metallurgical structure a substantial distance into said casting from all outer surfaces, physical soundness in terms of lack of substantial porosity, and fine-grain as-cast structure which materially increases the corrosion resistance of said article in comparison with the same composition cast in sand molds.

9. An annealed steel article of manufacture made according to the process of claim 6 characterized by smooth surfaces which accurately reproduce the surfaces of the refractory metal which forms the inner surface of the die, uniformity of metallurgical structure a substantial distance into said casting from its outer surfaces, physical soundness in terms of lack of substantial porosity, and fine-grain structure which materially increases the strength of said article in comparison with articles of the same composition cast in sand molds.

10. A process of claim 1 in which the ferrous casting solidifies at a rapid enough rate to permit ejection of said casting from said die in a time of less than about two seconds for castings having a maximum thickness of no more than about one-half inch.

References Cited UNITED STATES PATENTS 748,061 12/1903 Franklin 164-113 2,195,360 3/1940 Daesen 164-113 2,391,182 12/1945 Misfeldt 164-113 3,258,818 7/1966 Smith 164-138 X 3,286,312 11/1966 Davis et al. 249-1 14 X 3,435,880 4/1969 Goctz et al. 164-155 OTHER REFERENCES Precision Metal Molding, October 1965, TS 200.1 74,

Precision Metal Molding, February 1966, TS 200.P74,

J. SPENC E-R OVERI-IOLSER, Primary Examiner R. SPENCER ANNEAR, Assistant Examiner US. Cl. X.R. 

